Recently, hybrid cars and electric vehicles have been rapidly developed. Accordingly, braking of a vehicle has been rapidly developed from conventional mechanical braking toward electrical braking, and various proposals have been made.
In general, in order to electrically control a vehicle, a battery is used as its power supply. In this case, if electric power cannot be supplied from this battery due to some causes, the vehicle cannot be controlled with only this battery. Therefore, some configurations are equipped with an auxiliary battery as an auxiliary power supply in addition to the battery for the case of emergency.
As a prior art document with regard to this application, for example, Japanese Patent Unexamined Publication No. H05-116571 is known.
In this type of power supply apparatus for a vehicle, since an auxiliary power supply is responsible for braking of a vehicle in an emergency, it is extremely important to supply electric power from the auxiliary power supply reliably in an emergency. Therefore, it is important to surely predict the lifetime of the auxiliary power supply and to detect a state of the auxiliary power supply at any time.
However, herein, if a battery is used also as the auxiliary power supply, it is very difficult to predict the lifetime of the battery for the auxiliary power supply. Furthermore, as to the state of the battery, although a voltage can be checked, it is difficult to detect an abnormal state only from the voltage. Therefore, conventionally, the battery has been replaced by a new one earlier than usual so as to secure higher safety. However, even if the battery is replaced by a new one earlier than usual, it is difficult to check the state of the battery every day or every time the vehicle is started to move. Thus, further improvement in safety has been required.